Research on non-toxic inhibitors is of considerable interest in investigations into the replacement of hazardous classical molecules. This paper reports the action of four amino acids containing sulfur on the corrosion of mild steel in phosphoric acid solution with and without Cl-, F- and Fe3+ ions near and at the corrosion potential (E-corr) using both the polarization resistance method and electrochemical impedance spectroscopy (EIS). Both cysteine and N-acetylcysteine (ACC) showed higher inhibition efficiency than methionine and cystine. Adsorption of methionine onto a mild steel surface obeys the Frumkin adsorption isotherm and has a free energy of adsorption value (DeltaGdegrees(ads)) lower than those obtained in the presence of cystine, cysteine and ACC whose adsorption isotherms follow that of Langmuir. Both F- and Fe3+, ions stimulate mild steel corrosion while Cl- ions inhibit it. The binary mixtures of methionine, cysteine or ACC with Cl- or F- ions are effective inhibitors (synergism) while the combinations of the amino acid with Fe3+ or the ternary Cl-/F-/Fe3+ mixture have low inhibitive action (antagonism). EIS measurements revealed that the charge transfer process mainly controls the mechanism of mild steel corrosion in phosphoric acid solution in the absence and presence of the investigated additives. The mechanism of corrosion inhibition or acceleration is discussed.